The invention relates generally to restricting the flow of a fluid between two pressurized chambers and more specifically to a reverse flow tolerant brush seal for restricting a flow of fluid between pressurized chambers of a turbomachine.
Turbomachines, such as gas turbines and steam turbines, employ bladed rotors in a turbine section to convert thermodynamic energy from the fluids such as pressurized steam, compressed air and combustion gases into mechanical energy for rotating one or more centrally mounted shafts. The shafts, in turn, provide power to aircraft, heavy equipment, waterborne vehicles and electrical power generators. The interfaces between adjacent engine components in turbomachines are sealed in various ways to restrict leakage of fluids such as the pressurized steam, compressor air and combustion gases. There are many interfaces between rotating and stationary components in such turbomachines. Sealing these interfaces presents challenges due to the excessive fluid temperatures and pressures, combined with relative axial and/or radial movement between the engine components. Generally, sealing of these interfaces is done using various types of seals like labyrinth seals and honeycomb seals.
A brush seal is an advanced seal that provides an alternative to labyrinth or honeycomb seals. The seal is comprised of thousands of densely packed wire filaments (bristles) fused between two metallic plates. Bristles with a flexible end bridge a gap between adjacent components and any relative movement is absorbed through deflection of the bristles. Brush seals are very effective because they have minimum effective clearance during normal operation. The tortuous path through the bristles achieves the restriction effect even as the gap distance changes. Brush seals offer many advantages when compared with traditional seals. Unlike the labyrinth seal, a brush seal is designed to come in contact with a rotor to provide a positive seal.
Brush seal bristles are also susceptible to deflection due to fluid pressure loading. For this reason, back plates support the bristles along a majority of their length. The bristles are loaded against the back plate by the fluid pressure, thus preventing permanent deflection. The side plates may be scalloped where they contact the bristles to provide a space for bristle flexure and to allow any frictional heat to dissipate out of the bristles.
But previous fleet experience shows that brush seals are ineffective during the reverse flow operations, such as startups, due to lifting of bristles and thereby opening up of clearances. Brush seals are designed to have the bristles continuously loaded in one direction, against the back plates. Brush seals are most effectively used in applications where a continuous pressure differential exists. If a brush seal is installed in reverse or an unanticipated flow reversal occurs, the unsupported bristles will deflect under pressure. The bristles get lifted up since there is no plate to support bristles in reverse flow direction.
Bristle deflections eventually yield the bristle ends, reducing their sealing effectiveness and rendering them unacceptable for continued service. Reduced brush seal effectiveness will increase fluid leakage, fuel usage and, consequently, increase operating costs until the brush seal is replaced. Removal and disassembly of a turbomachine for brush seal replacement is both costly and time consuming.
FIG. 1 illustrates a radial side sectional view of a prior art brush seal 10 for sealing a rotating shaft. The brush seal 10 includes a housing 15 for mounting a brush holder 20. The housing includes a front plate and a backing plate for seating brush seal bristles 22. The bristles 22 are seated against the support surface 35 or 31 of the back plate 30 when a higher pressure P1 is present in a first chamber 40 on one axial side of the brush seal 10 relative to the pressure P2 in a second chamber 45 on the second axial side of the brush seal. The housing is positioned to support the bristles in proximity to movable shaft 50. The brush seal bristles 22 are held in position against surface 55 of the movable shaft 50 to minimize the leakage flow 60 created by this pressure differential.
Experience shows that flow reversals occur under certain turbomachine operating conditions, thus precluding the use of brush seals in certain applications. FIG. 2 illustrates a radial side sectional view of a the prior art seal brush when, due to an operating condition of the turbomachine, pressure P2 of the second chamber 45 on the second axial side of the brush seal is higher than the pressure P1 of the first chamber 40 on the first axial side. The pressure differential forces a lower end of the bristles 22 off the support surface 35 of backing plate 30 increasing clearance 70 between the end of the bristles 22 and the surface 55 of the moving shaft 50. The increased clearance 70 allows a reverse flow 65 much larger than desired, resulting in a commensurate loss of high-energy fluid and efficiency of the turbomachine.
For steam turbines, brush seals are very effective at providing sealing during normal operation with sealing steam flow oriented in the one direction for the installed brush seals. However, during reverse flow conditions such as startup, shutdown, trips and operation on the turning gear, the brush seal may be ineffective at sealing the reverse flow, hence requiring more auxiliary steam and a larger auxiliary boiler size to provide sealing steam.
Attempts have been made to improve brush seal effectiveness such as Addis in US2008/0203671 by employing at least two in-series brush seal stages with a mechanism to allow the high pressure fluid to bypass an upstream bristle stage and properly load the downstream bristle stage against a backplate. Such a mechanism may be costly and undesirable due to the need for two brush seal stages. Also it requires a longer axial space, which will increase the bearing span.
Accordingly, it would be desirable to provide a brush seal that is tolerant of flow reversals.